This invention relates to a starter motor for an engine for a vehicle such as a motorcycle or the like and to a method for starting such engines and reducing starter noise at the time of the starting operation.
In an engine for a vehicle such as a motorcycle, a starter motor is used at the time of engine starting to rotate an engine shaft under battery power. The starter motor is generally a DC motor driven through a relay by operation of a manual switch such as main switch or a starter switch of the vehicle. In this case, the switch is first turned ON to rotate the starter motor, for cranking. When the engine is started after the starter motor load drops to zero, resulting in a maximum rotational speed. At this moment, the switch is turned OFF to stop the power supply to the starter motor. As a result, the starter motor output shaft is disconnected from the engine shaft by a one way clutch type of device and it rotates idly by its inertia and stops after gradually reducing its rotational speed.
At the time of the engine starting described above, during the time while the starter motor is stopping its rotation after the power supply to the starter motor is OFF it generates an abnormal and unpleasant noise. The cause of this phenomenon may be understood by reference to FIG. 1. In this figure the horizontal axis represents time and the vertical axis represents both starter motor rotational speed and noise level.
The power source of the starter motor is turned ON at a time t0 and begins to rotate for cranking. When the engine is started, the starter motor load drops to zero and the rotational speed increases to a maximum. At a time t1 when this state is reached, the starter switch is turned OFF by hand.
As a result, the starter motor rotates idly as a result of its inertia, decreasing its rotational speed gradually and stopping eventually at a time t2. Between the times t1 and t2, an abnormally high noise is generated. The noise at this point is an abnormal and unpleasant one and unusually is louder than the engine noise or even that of the starter motor during the actual starting operation.
This abnormal noise is caused by the starter motor yoke of its stator resonating when the natural frequency of the yoke coincides with the number of times of cogging reaction at a specific motor speed. This resonance frequency corresponds to a frequency determined by the least common multiple of the number of slots of an armature and the number of magnetic poles of magnets, or the cogging number/rotation, and the cogging reaction produced at a specific motor speed. In an actual measurement shown in FIG. 1, the cogging number is the least common multiple of 28 for a motor with fourteen slots and four-pole magnets. An abnormal noise is generated at the time of the rotational speed of 5100 rpm. In this case, the resonance frequency is expressed as follows:
(28xc3x975100/60)xc3x972=4760 Hz. 
More specifically, an armature connected to the output shaft of the starter motor is formed of a plurality of radially disposed cores. Electrical coils are wound on these cores and face a plurality of magnets on the inside surface of the starter motor yoke. The armature is rotated through successive attractions of magnetic forces of the magnets.
When the cores of the armature pass across the magnets and its polarities are changed, the armature changes its rotational torque, generating cogging with a perturbed movement. Therefore, the larger the magnetic forces are, the greater cogging is generated, resulting in an abnormal noise due to the reaction.
Normally the material of the permanent magnets is a ferrite-based magnetic material. However, neodymium-based magnets made from a magnetic material containing Nd of a rare metal element or its compound known as high-energy magnets are preferred because they permit a higher output starter motor for a given size. If such neodymium-based magnets are used, since the magnetic forces are great, the problem of an abnormal noise due to the cogging reaction is amplified.
It is therefore a principle object of this invention to provide an improved starter motor arrangement and method of starting an internal combustion engine that reduces noise during the starting operation.
A first feature of this invention is adapted to be embodied in a starter arrangement for an internal combustion engine. The starter arrangement comprises a DC electrical motor having an output shaft in starting arrangement with a shaft of the engine for starting the engine. A battery is provided for selectively energizing terminals of the DC electrical motor for driving the engine shaft to start the engine. A braking arrangement brakes the rotation of the starter motor output shaft when the engine starts.
In a preferred embodiment of this first feature, the braking arrangement comprises a switching arrangement that connects the terminals of the DC electrical motor to the battery to charge the battery upon the deenergization of the terminals for stopping the driving of the DC electrical motor by regeneratively braking the rotation of said DC electrical motor.
Another feature of the invention is adapted to be embodied in a method for starting an engine with a DC electrical motor and reducing starter motor noise. A DC electrical motor has an output shaft in starting arrangement with a shaft of the engine for starting the engine. A battery is also provided. The method comprises the steps of selectively energizing terminals of the DC electrical motor for driving the engine shaft to start the engine upon operator demand and the rotation of the starter motor output shaft is braked when the engine starts.
In a preferred embodiment of this other feature the starter motor output shaft is braked by connecting the terminals of the DC electrical motor to the battery to charge the battery upon starting of the engine to regeneratively brake the rotation of the DC electrical motor.